Good for you for working your way through baby sweaters and on to adult sizes. These kinds of directions are often confusing. You're going to dec at the neck edge on the first shoulder and bind off at the beginning of the row for the shoulder shaping at the same time. Later you'll get to shaping the other shoulder.
Start with the cast off of 4 sts (size small) on the RS row, work till you have 22sts on the right needle, turn. Don't worry about the unworked sts. You'll get to them later. On the WS row, work one stitch, dec 1, and finish the row. You'll have 21 sts for that shoulder remaining.
You're now onto a RS row, so cast off 4 sts, work to the last 3 sts, dec 1, work 1 and you should be at the end of the RS row. Now turn, work 1 st, dec, and work to the end of the row.
You'll continue this way until you've completed all the decs and bind offs. Sometimes I make a small diagram showing these so that I can check them off as I go along.